Field of the Invention
The present invention relates to a grinder pump of the water immersed type which functions both as pump and grinder for the purpose of pumping liquid and at the same time grinding or shredding foreign material contained in the flowing liquid.
As is well known, a grinder pump is provided with a grinding or shredding mechanism at the suction port of the pump or adjacent the pump impeller. As to the pump section various types of pumps such as vortex type pump, spiral type pump (semi-open type pump, closed type pump), snake type pump and others have been hitherto employed, whereas as to the grinding section both a single stage grinding mechanism and double stage grinding mechanism have been proposed and put in practical use.
One of the conventional spiral type pumps is disclosed in U.S. Pat. No. 3,961,758 assigned to Peabody Barnes Inc. In this prior invention the impeller is a semi-open type and the grinder is so constructed that foreign material is ground or shredded with the aid of a single stage grinding mechanism comprising a stationary cylinder and a rotary cylinder adapted to be rotated in said stationary cylinder together with grinding wheels while the pumped liquid containing foreign material therein passes through the space between the stationary cylinder and rotary cylinder. In this prior art, the shallow conical shaped impeller is short in axial length and, thus, in this type of a single stage, it is quite difficult to cut elongated foreign material such as polyvinyl chloride threads or the like entering the clearance and these foreign materials may clog the axial grooves serving as a cutter.
In another prior invention under U.S. Pat. No. 3,866,841 assigned to Elepon Technical Center Kabushiki Kaisha a submersible pump is disclosed which contains a single stage grinding structure comprising a closed type impeller. The grinding structure effects cutting and shearing by means of a combination of a stationary cutter and a rotary cutter arranged in horizontal planes, both cutters being relatively long.
U.S. Pat. No. 3,973,866 assigned to Vaughan Co., Inc. discloses a centrifugal slurry pump which comprises an impeller with wide blades serving also as cutter blades. In this slurry pump cutting or grinding is effected in the horizontal direction by means of the cutter in a single stage, while agitation is effected by means of agitating blades arranged in front of the suction port. However it has a drawback in that foreign material may clog the space between the suction port and pump impeller to block the rotation of the impeller.
Next, U.S. Pat. No. 3,417,929 discloses a comminuting pump which is provided with a single stage grinding mechanism comprising a plurality of spiral grinding blades which spirally extend in the axial direction as an extension of the impeller blades.
Now, a few examples of centrifugal pumps having a double grinding mechanism incorporated therein will be briefly described below.
In U.S. Pat. No. 3,650,481 assigned to Hydro-O-Matic Pump Co. a grinding pump is disclosed containing a semi-open type impeller. Due to the fact that the impeller blades of this pump are short, foreign material flown therethrough is cut to smaller size in both the vertical and horizontal directions so as to prevent any occurrence of clogging in the piping as well as in the impeller. Also agitating blades disposed before the cutter and axial blades disposed after the cutter improve the passage of foreign material. A similar grinding pump is disclosed in U.S. Pat. No. 3,726,486.
In U.S. Pat. No. 4,143,993 granted to A. Blum, grinding and cutting are effected in a different manner. Specifically, cutting is effected by means of a single cutter separately disposed below the impeller, while grinding is effected by means of the latter, the cutter and impeller being located in vertical alignment. The cutter contains a wave-shaped circular cutting edge therearound.
As described above, the conventional grinder pump is so constructed that the grinder section is located at the pump inlet and the rotary cutting blades are adapted to rotate inside of the stationary cutter so that pumped liquid containing foreign material to be treated passes therebetween. However it has been found with the conventional grinder pump that foreign material to be shredded, for instance, fibrous foreign material gets entangled in the clearance between the blades of the semi-open impeller and the casing end wall at the suction port or around the blades or the like and thereby clogging takes place, resulting in stoppage of rotation. In order to eliminate the drawback as described above a secondary grinding mechanism was introduced together with an agitating blade in front of the pump inlet. In case that the secondary grinding mechanism is provided, however, the cutter tends to jam easily.
Also, because the whole unit is complicated, further problems such as rotation blocking derived from the complicated structure, increased torque, time-consuming assembly and disassembly and other, arise. Further, since the agitating blade is separately disposed, it is possible that excessive scattering of foreign material may occur before it is to be cut in the grinder section.
In some case it happens that long and hard foreign material such as wire or the like clogs the vicinity of the cutter section in the grinder.
As is readily understood from the above description, the grinder pumps of the prior art are of various designs and each type has its inherent specific problems or drawbacks. They all have the common object of grinding or shredding foreign material included in the pumped liquid so the liquid will thereafter pass smoothly through the grinder pump unit, but it is natural that the characteristic features of the grinding mechanism should vary depending on the mechanical or physical properties of the foreign material to be treated.